1. Field of the Invention
The present invention relates to an apparatus for measuring the surface profile of aspherical photographic lenses and other aspherically surfaced optical components. More specifically, the present invention relates to an apparatus for measuring the profile of an aspherical surface by the polar coordinate system in which the aspherical surface to be analyzed is rotated about an axis that passes through the center of a reference spherical surface approximate to the aspherical surface and which is perpendicular to the optical axis of said aspherical surface. The profile of the aspherical surface of interest is measured based on the resultant angle of rotation and the amount of deviation from the radius of rotation.
2. Prior Art of the Invention
The principle of measurement of the profile of an aspherical surface by the polar coordinate system is illustrated in FIG. 1. An aspherical surface 1 to be analyzed is rotated about an axis 3 that passes transversely through the center of curvature of an approximate reference spherical surface 2 with a radius of curvature R and which is perpendicular to the optical axis of the aspherical surface 1. The profile of the aspherical surface 1 of interest is determined on the basis of measurement of the angle of rotation .theta. and the amount of deviation .DELTA.R between the profile of the surface 1 and the surface Z along the radius of rotation. A prior art apparatus that is operated on this principle is shown schematically in FIG. 2.
The aspherical surface 1 to be analyzed (hereinafter simply referred to as the surface of interest) is mounted in such a way that the center of curvature of a reference surface that approximates the aspherical surface 1 of interest is in alignment with the axis of rotation 3 of a rotating means 4. When the surface of interest is rotated about a transverse axis 3 by an angle .theta., a certain amount of deviation .DELTA.R from the radius of rotation occurs. This deviation .DELTA.R is sensed as the amount of movement of a mechanical feeler 6 that is guided by a bearing 5 while being kept in biased contact at one end with the aspherical surface 1. The other end of the feeler 6 is provided with a reflective mirror 7, such as a cat's eye mirror, that is designed to move the same distance as that traveled by the feeler 6. The bearing 5, feeler 6 and reflective mirror 7 attached to the feeler 6 constitute the principal components of a contact probe 8.
Detection of the amount of deviation .DELTA.R from the radius of rotation is achieved by a laser length metering system 9 operating by the heterodyne interference method. A suitable laser length metering system operating on the principle of heterodyne interference may be selected from among the products of Hewlett-Packard Company, say Model HP 5528A. This model is known to be the most convenient and reliable length metering apparatus available today.
Both the angle .theta. and the deviation .DELTA.R are detected by the rotating means 4 and the length measuring system 9, respectively, to thus provide a measurement of asphericity. The measurement of the angle .theta. may be done by an angular encoder attached to a slowly turning motor driving a turntable on which the aspherical surface rests.
The operating principle of the laser length metering system 9 is as follows. Two beams of light emerging from a Zeeman laser 10 that are polarized in directions transverse to each other and which have slightly different frequencies f.sub.1 and f.sub.2 are incident on a beam splitter 11 from which part of the light is separated and detected with a low frequency photo detector 12 at a beat frequency f.sub.1 -f.sub.2.
While the beam splitter 11 transmits the remaining components of light, one component having the frequency f.sub.2 is guided through a polarizing beam splitter 13 to encounter a reference cat's-eye mirror 14, from which it is reflected and falls upon and is detected by a low frequency photodetector 15 as reference light. The other component having the frequency f.sub.1 falls upon a reflective mirror 7 after passing through the beam splitter 11. The frequency of the light reflected from the reflective mirror 7 is Doppler-shifted from f.sub.1 to f.sub.2 +.DELTA.f by the instantaneous velocity of the displacement of the mirror 7 that results in the displacement .DELTA.R. The time integral of the Doppler shift is indexed by the rotation angle as the aspherical surface 1 is turned to indicate the amount of deviation of the aspherical surface 1 of interest. As a result of interference by the reference light, the light detected at the photodetector 15 has a beat frequency equal to f.sub.1 -f.sub.2 +.DELTA.f.
The outputs of the two detectors 12 and 15 are counted by peak or zero-crossing counters 31 and 32 and these counts are differenced in a subtracter 33. The output of the subtracter 33 is a time integral of .DELTA.f, i.e. .DELTA..PHI., and thus is proportional to .DELTA.R, resulting from the rotation of the aspherical surface 1.
A calculation circuit 34 converts between .DELTA..PHI. and .DELTA.R dependent upon the frequency f.sub.1 or f.sub.2. This deviation .DELTA.R is paired with the measured angle .theta. from the turning means 4.
The prior art apparatus of the type contemplated by the present invention has the advantage that it ensures a very high precision in measurement and that it is capable of achieving profile measurement of a non-specular surface. On the other hand, this apparatus is not suitable for measurement of the profile of a finished surface or other vulnerable surfaces such as those of finished products.